This invention relates to solid state lasers and, more particularly, to such lasers in which transverse modes are coupled to form optical filaments that beat with one another so as to modulate the laser output, typically at terahertz (THz) frequencies.
There has been considerable interest in recent years in the development of sources of optical radiation that can be modulated at THz frequencies. These sources can be used for submillimeter spectroscopy, nondestructive internal imaging of opaque materials, and optical communications systems (especially those requiring low timing jitter pulse trains). One approach is to mix two laser radiation fields that differ in frequency by the desired THz frequency. Mixing is typically performed within a semiconductor amplifier or a fiber amplifier. The difference frequency is produced by nonlinear mixing within the amplifier gain material. Alternatively, mixing can be accomplished using a GaAs photoconductive mixer-antenna structure.
In some cases the outputs of two separate laser sources have been mixed. See, for example, McIntosh et al., Appl. Phys. Lett., Vol. 67, No. 26, pp. 3844-3846 (1995), which is incorporated herein by reference. This approach has the disadvantage that the coherence of the mixing is subject to independent fluctuations from both source lasers. To address this limitation, other researchers have used a single laser source to produce both frequencies to be mixed; i.e., passive harmonic mode locking of a diode laser to create THz modulated radiation for use in driving photomixers to generate THz energy. See, for example, Arahira et al., Optics Lett., Vol. 19, No. 11, pp. 834-836 (1994), which is incorporated herein by reference.
In one case a conventional multimode DBR laser was used to produce a number of longitudinal mode frequencies. Two of these frequencies were selected for mixing by selective filters that separated the desired frequencies from the multimode output spectrum. Since the longitudinal mode spacing of this DBR laser was 110 GHz, beat frequencies in increments in excess of 110 GHz could be produced. Mixing at 1.34 THz was demonstrated. See, for example, Pelusi et al., Appl. Phys Lett., Vol. 71, No. 4, pp. 449-451 (July, 1997), which is incorporated herein by reference.
In yet another design, a two-grating DBR laser was used to limit the longitudinal mode spectrum to only two modes. In this case, the mode separation was 163.5 GHz. These two modes were then mixed in a GaAs photoconductive mixer-antenna to produce radiation modulated at 0.1635 THz. See, for example, Gu et al., Proceedings of CLEO, pp. 261-262 (May, 1998), which is incorporated herein by reference.
None of these laser optical sources has relied on the existence of optical filaments in order to modulate the radiation output at terahertz frequencies. In the context of solid state lasers that include a planar optical waveguide, the term filament as used herein means an intracavity, in-plane (i.e., in the plane of the waveguide) intensity distribution of the lasing radiation that exhibits a meandering (e.g., sinusoidal) pattern of nodes and peaks that weaves from one side of the waveguide cross-section to another (or from the top to the bottom of the waveguide cross section) along the longitudinal axis of the laser. A few prior art lasers have exhibited such filaments; e.g., 0.98 xcexcm pump laser diodes and 1.3 xcexcm buried heterostructure (BH) laser diodes investigated by Ohkubo et al., Jpn. J. Appl. Phys., Vol. 35, pp. L34-L36 (1996) and Schemmann et al., Appl. Phys. Lett., Vol. 66, No. 8, pp. 920-922 (1995), both of which are incorporated herein by reference. But prior art workers have considered filamentation in these lasers to be undesirable because the maximum useful output power is limited by the lateral beam deflection that occurs when the filament forms. In addition, the authors did not appreciate the way such filaments might be used to generate THz modulation.
In accordance with one aspect of the invention, a self-modulated, solid state laser comprises an intracavity optical waveguide that supports a multiplicity of lasing filaments each at a different optical frequency. At least two of the filaments temporally beat with one another so as to modulate the intensity of the laser output. In accordance with one embodiment of the invention, the waveguide supports a multiplicity of transverse modes, and the laser includes a mode mixing mechanism that mixes the energy of at least two pairs of the modes, each coupled pair generating a separate filament. In an illustrative embodiment, the filaments are mode-locked and the laser output is modulated at frequencies on the order of 1 THz.